This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-368606, filed on Dec. 4, 2000; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electroabsorption modulator and a fabricating method thereof, in particular to an electroabsorption modulator appropriate for use in high speed modulation, 40 GHz or more, of laser light from a semiconductor laser, and a fabricating method thereof.
2. Description of the Related Art
Recently, as information communication demand in the Internet or the like increases, technology for high speed transmission of a larger volume of information to a distant place, not to mention to a trunk line, but also to a branch line, is demanded. As such technology, there is a large-capacity optical communication system.
In the large-capacity optical communication system, light emitted from a semiconductor laser is speedily modulated into digital signal; the modulated light is transmitted by means of the optical fiber; thereby high speed and distant transmission is realized. As an optical modulation method, it is general to directly modulate a semiconductor laser. However, the direct modulation of the semiconductor laser causes a relaxation oscillation in the semiconductor laser, thereby resulting in variation of wavelength due to chirping. When such light is transmitted through the optical fiber, there occurs a difference between the transfer times of the optical fiber, resulting in mode dispersion. As a result, communication distance is limited.
As a method for reducing the chirping at the direct modulation, there is a method in that light emitted from the semiconductor laser undergoes external modulation by means of an optical modulator. Among such optical modulators, an electroabsorption modulator (EA modulator) is in heavy usage. The electroabsorption modulator makes use of the quantum confinement Stark effect of a quantum well, and has advantages in that it may be mass-produced at relatively low costs, and may be driven at low voltages.
FIG. 13A is a perspective view showing a rough configuration of an existing electroabsorption modulator, and FIG. 13B is a sectional view obtained by cutting along a C-D line in FIG. 13A. In FIG. 13A, the electroabsorption modulator is provided with an optical modulation region MA of a length Lm and optical coupling regions CA, which are formed on both sides of the region MA; in the optical modulation region MA and the optical coupling regions CA, a mesa, which is formed in stripe in a light incident direction (A-B direction), and grooves MZ, which are formed on both sides of the mesa MS, are disposed. The length Lm of the optical modulation region MA may be set at, for instance, 100 xcexcm.
In the optical modulation region MA, as shown in FIG. 13B, an n-InP cladding layer 62 is formed on an n-InP substrate 61; and in the mesa MS portion, an optical absorption layer 63, a p-InP cladding layer 64, and a p-InGaAs contact layer 65 are formed. The optical absorption layer 63 has a multiple quantum well (MQW) structure, and may be formed by combining 14 pairs of, for instance, an InGaAsP quantum well layer of 1.5 nm and an InGaAsP barrier layer of 1.3 nm.
Furthermore, a silicon oxide film 68 is formed in the mesa MS and the grooves MZ, and resin 69 is filled in the grooves MZ. An n-side electrode 70 is formed on a back-face of the n-InP substrate 61, and a p-side electrode 71 is formed on the mesa MS of the optical modulation region MA.
Meanwhile, in the optical coupling region CA, as shown in FIG. 13A, the n-InP cladding layer 62 is formed on the n-InP substrate 61, and, in the mesa MS portion, an InGaAsP guide layer 66 and an n-InP cladding layer 67 are formed. Furthermore, the silicon oxide film 68 is formed in the portions of the mesa MS and the grooves MZ, and the resin 69 is filled in the grooves MZ. A composition of the InGaAsP guide layer 66 may be set so that a wavelength of, for instance, 1.1 xcexcm may be obtained.
Light inputted in the optical coupling region CA is transferred through the InGaAsP guide layer 66 to the optical modulation region MA. Upon the light being transferred to the optical modulation region MA, the Stark effect is generated in the optical absorption layer 63 based on a voltage applied to the p-side electrode 71, and an energy gap in the quantum well varies. When the energy gap varies, an optical absorption wavelength due to exciton varies, and transmittance of the laser light in the optical absorption layer 63 varies, thereby optical modulation is performed. The modulated light is emitted through the optical coupling region CA.
FIG. 14A through FIG. 16B are diagrams showing a sequence of fabricating an existing electroabsorption modulator. FIG. 14A, FIG. 14B, and FIG. 14D are sectional views obtained by cutting along an A-B line in FIG. 13A, FIG. 15B and FIG. 16A sectional views cut along a C-D line in FIG. 13A, and FIG. 15C and FIG. 16B sectional views cut along an E-F line in FIG. 13A.
In FIG. 14A, the n-InP cladding layer 62, the optical absorption layer 63, the p-InP cladding layer 64, and the p-InGaAs contact layer 65 are successively grown on the n-InP substrate 61, by means of MOCVD (metal-organic chemical vapor deposition).
Next, as shown in FIG. 14B and FIG. 14C, a silicon oxide film 72 of a width Lm is formed on the p-InGaAs contact layer 65, and etching, such as RIE, is performed with the silicon oxide film 72 as a mask, thereby the optical absorption layer 63, the p-InP cladding layer 64, and the p-InGaAs contact layer 65 in the optical coupling regions CA are removed.
Next, as shown in FIG. 14D, an InGaAsP guide layer 66 and an n-InP cladding layer 67 are selectively grown on the optical coupling regions CA, by performing deposition such as MOCVD with the silicon oxide film 72 as a mask. Then, a silicon oxide film 73 is deposited on an entire surface by means of CVD and so on. By depositing the InGaAsP guide layer 66 in the optical coupling region CA, the InGaAsP guide layer 66 and the optical absorption layer 63 may be allowed to optically couple.
Next, as shown in FIG. 15A, FIG. 15B, and FIG. 15C, the silicon oxide film 73 is patterned into stripes corresponding to the mesa MS and the grooves MZ. Then, by performing chemical etching with the patterned silicon oxide film 73 as a mask, the optical absorption layer 63, the p-InP cladding layer 64, and the p-InGaAs contact layer 65 in the grooves MZ of the optical modulation region MA are removed in mesa; and the InGaAsP guide layer 66 and the n-InP cladding layer 67 in the grooves MZ of the optical coupling region CA are removed in mesa. Thereby, the optical absorption layer 63 of the mesa MS is separated by the grooves MZ.
The length Lm of the optical modulation region MA may be set at, for instance, 100 xcexcm; a width LS of the mesa MS at, for instance, 5 xcexcm; and a width Lp of the optical absorption layer 63 at, for instance, 2 xcexcm. Optical modulation is performed in the optical absorption layer 63 of the mesa MS corresponding to the stripe portion.
Next, as shown in FIG. 16A and FIG. 16B, after removing the silicon oxide film 73, the silicon oxide film 68 is deposited on an entire surface by means of CVD and so on, and the resin 69 is filled in the grooves MZ. Then, the p-side electrode 71 is formed on the mesa MS portion of the optical modulation region MA, and furthermore, a bonding pad 74 is formed. Thereafter, the n-InP substrate 61 is ground to substantially 100 xcexcm, and the n-side electrode 70 is formed on a back-face of the n-InP substrate 61.
A cut-off frequency of the optical modulator depends on element capacitance and element resistance. In order to allow the optical modulator to operate at high-speeds, the element capacitance is designed to be as small as possible. The element capacitance mainly depends on parasitic capacitance of the bonding pad 74 and PN junction capacitance in the stripe portion of the optical modulation region MA. The parasitic capacitance of the bonding pad 74, though depending on relative permittivity of the resin 69 and a film thickness of the silicon oxide film 68, may be decreased to from 10 to 50 fF depending on design.
Meanwhile, the capacitance of the stripe portion of the optical modulation region MA is 150 fF per 100 xcexcm, larger by substantially one figure than the parasitic capacitance of the bonding pad 74. Accordingly, in order to reduce the element capacitance of the optical modulator, it is effective to shorten the length Lm of the optical modulation region MA.
However, when the length Lm of the optical modulation region MA is made shorter, though the element capacitance of the optical modulator becomes smaller, an area of the p-InGaAs contact layer 65 also becomes smaller. As a result, when the element capacitance of the optical modulator is made smaller, the element resistance becomes larger, resulting in a problem in that the high-speed operation of the optical modulator becomes difficult.
The element resistance of the optical modulator, shown in FIG. 13A, is, for instance, substantially 20 xcexa9, substantially threefold or more larger in comparison with that of the semiconductor laser that has an element length 300 xcexcm. Accordingly, in the existing optical modulator, the cut-off frequency becomes substantially 28 GHz, resulting in difficulty in operating with high-speed of 40 GHz.
The object of the present invention is to provide an electroabsorption modulator capable of realizing a high-speed operation of the optical modulator, and a fabricating method thereof.
An electroabsorption modulator according to an embodiment of the present invention includes a first conductivity type cladding layer formed on a first conductivity type substrate: an optical absorption layer formed on the first conductivity type cladding layer: a second conductivity type cladding layer formed on the optical absorption layer: a contact layer formed on the second conductivity type cladding layer: and a high-resistance layer partially formed in a region in an optical propagation direction of the second conductivity type cladding layer.
Furthermore, an electroabsorption modulator according to another embodiment of the present invention includes a first conductivity type cladding layer formed on a first conductivity type substrate: an optical absorption layer formed on the first conductivity type cladding layer: an optical waveguide layer, which is formed on the first conductivity type cladding layer so as to optically couple with the optical absorption layer, and has a stripe width larger than that of the optical absorption layer: a second conductivity type cladding layer formed on the optical absorption layer and the optical waveguide layer: a contact layer formed on the second conductivity type cladding layer: and a proton implanted layer obliquely formed on the second conductivity type cladding layer.
A fabricating method of an electroabsorption modulator according to an embodiment of the present invention includes forming a first conductivity type cladding layer on a first conductivity type substrate: forming an optical absorption layer on the first conductivity type cladding layer: forming an optical waveguide layer, which is optically coupled with the optical absorption layer, on the first conductivity type cladding layer: forming a second conductivity type cladding layer on the optical absorption layer and the optical waveguide layer: forming an oxidizable semiconductor layer in a region corresponding to on the optical waveguide layer in the second conductivity type cladding layer: forming a contact layer on the second conductivity type cladding layer: etching the contact layer, the second conductivity type cladding layer, the oxidizable semiconductor layer, the optical absorption layer and the optical waveguide layer in stripes: and oxidizing the oxidizable semiconductor layer from a sidewall of the stripe.
A fabricating method of an electroabsorption modulator according to another embodiment of the present invention includes forming a first conductivity type cladding layer on a first conductivity type substrate: forming an optical absorption layer on the first conductivity type cladding layer: forming an optical waveguide layer, which is optically coupled with the optical absorption layer, on the first conductivity type cladding layer: forming a second conductivity type cladding layer on the optical absorption layer and the optical waveguide layer: forming a contact layer on the second conductivity type cladding layer: etching the contact layer, the second conductivity type cladding layer, the optical absorption layer, and the optical waveguide layer in stripes, so as for a stripe width of the optical absorption layer to be larger than that of the optical waveguide layer: and implanting protons in an oblique direction from a sidewall of the stripe.